It's a basic filter, with reliable properties that indicate that it's a decent predictor for a quality home that a shelter is seeking. If not using a veterinarian reference, which is a subjective reference from a qualified professional, what, exactly, are shelters supposed to use to filter through homes to ensure a good match? Home checks are going so far - they don't actually speak to the care the dog itself will receive and personal references are pretty much useless.
Also, by not spaying females, you are running the risk of pyometra, a serious, and life-threatening infection that has killed two dogs that I know of personally. Sorry, but I'm not running that kind of risk.